To understand which answer choice is correct, one must first compare two related concepts, diffusion and osmosis. Diffusion is defined as movement of particles from an area of a higher concentration to an area of a lower one. This process is typically seen when ions or molecules diffuse across a cell membrane in an attempt to make the concentration on either side of the membrane equal. It is important to note that diffusion occurs without...

To understand which answer choice is correct, one must first compare two related concepts, diffusion and osmosis. Diffusion is defined as movement of particles from an area of a higher concentration to an area of a lower one. This process is typically seen when ions or molecules diffuse across a cell membrane in an attempt to make the concentration on either side of the membrane equal. It is important to note that diffusion occurs without requiring energy. Osmosis is a special form of diffusion used specifically for the case of water moving across a semi-permeable membrane. This is important in cells because a small molecule like water can easily diffuse across a cell's membrane. This process can produce internal or external forces on the cell that can lead to a number of different cellular conditions. This creates environments that are either:

Hypertonic, a state in which less water is outside the cell, leading to cell shrinkage due to water leaving.

Hypotonic, a state in which more water is outside the cell, causing water to enter the cell and leading to expansion and possibly rupture of the cell, resulting in cell death.

Isotonic, a state in which water on the inside and outside of the cell becomes equal, and no net movement occurs.

Looking specifically at this question, one will notice that answers A, B, and C all deal with the movement of other small molecules besides water, or in the case of C ions, across a semi-permeable membrane. In these cases, all of these examples are examples of diffusion. Only D, the example of water moving into the root cells of plants, is an example of osmosis. Hope this helps!